> ## Documentation Index
> Fetch the complete documentation index at: https://avala.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Semantic Segmentation Tool

> Create pixel-level masks for semantic or instance segmentation with brush, eraser, fill, and SAM assist

The Segmentation tool creates pixel-level masks for semantic or instance segmentation. It includes four sub-tools: Brush, Eraser, Fill, and SAM Assist.

## When to Use

Use segmentation when:

* You need pixel-precise object boundaries
* The task requires semantic segmentation (every pixel labeled) or instance segmentation (separate mask per object)
* Objects have complex, irregular shapes that polygons cannot capture efficiently
* You are training models like DeepLab, U-Net, Mask R-CNN, or similar architectures

Consider a different tool when:

* You only need rough localization -- use the [Bounding Box](/docs/annotation/guides/tools/bounding-box) tool
* Object boundaries are simple enough for polygon outlines -- use the [Polygon](/docs/annotation/guides/tools/polygon) tool (faster for simple shapes)
* You need 3D annotations -- use the [3D Cuboid](/docs/annotation/guides/tools/cuboid-3d) tool

## Usage

1. Press `M` or select the **Segmentation tool** from the toolbar
2. Choose a label from the label panel
3. Use one of the sub-tools:

### Brush

* Click and drag to paint a mask region
* Adjust brush size with `[` (smaller) and `]` (larger)
* Hold `Shift` to paint in a straight line between two clicks

### Eraser

* Press `X` to toggle eraser mode
* Click and drag to erase mask pixels
* Same size controls as the brush

### Fill

* Click inside an enclosed region to flood-fill it with the current label
* Useful for filling the interior of a polygon outline quickly

### SAM Assist

* Press `S` to activate SAM mode within the segmentation tool
* Click on the object to generate a mask automatically
* Refine with positive clicks (include region) and negative clicks (exclude region)
* Press `Enter` to apply the mask

## Shortcuts

| Shortcut               | Action                                |
| ---------------------- | ------------------------------------- |
| `M`                    | Activate the Segmentation tool        |
| `S`                    | Switch to SAM assist mode             |
| `X`                    | Toggle eraser mode                    |
| `[` / `]`              | Decrease / Increase brush size        |
| `H`                    | Toggle mask visibility                |
| `Shift` + click        | Paint straight line to click location |
| `Enter`                | Apply SAM mask                        |
| `Delete` / `Backspace` | Delete selected mask                  |
| `Ctrl+Z` / `⌘Z`        | Undo                                  |
| `Ctrl+Shift+Z` / `⌘⇧Z` | Redo                                  |

## Recommended Workflow

The fastest approach for most objects is:

1. **Start with SAM**: Click on the object to get an initial mask
2. **Refine with positive/negative clicks**: Add or remove regions as needed
3. **Apply the mask**: Press `Enter`
4. **Clean up edges**: Switch to the brush or eraser at a small size for precise edge cleanup
5. **Zoom in**: Use `+` or scroll to zoom into boundary areas for fine detail work

## Common Mistakes

* **Jagged edges from large brush size**: Use a smaller brush near object boundaries for cleaner results
* **Missing thin features**: Thin parts of objects (legs, antennae, poles) are easy to miss -- zoom in to verify
* **Overlapping labels**: When two labels overlap, the last painted label takes priority -- be intentional about overlap order
* **Not using SAM**: Manually painting an entire mask from scratch is much slower than starting with SAM and refining

## Advanced Tips

* Start with SAM for the initial mask, then refine edges with the brush and eraser
* Use the eraser at a small size for precise edge cleanup
* Zoom in (`+` or scroll) for fine detail work along object boundaries
* Each label gets a distinct color overlay, configurable in the label panel
* Toggle mask visibility with `H` to see the underlying image and verify boundaries
* For batch labeling of similar objects, SAM assist can dramatically speed up the process

<Tip>
  Segmentation masks are stored as run-length encoded (RLE) data for efficient storage. The viewer handles encoding and decoding automatically.
</Tip>

## Related Tools

* [Polygon](/docs/annotation/guides/tools/polygon) -- For vector-based outlines (faster for simple shapes)
* [Bounding Box](/docs/annotation/guides/tools/bounding-box) -- For quick rectangular localization
* [Classification](/docs/annotation/guides/tools/classification) -- For image-level labels without geometry
* [Keypoint](/docs/annotation/guides/tools/keypoint) -- For pose estimation skeletons